1. Field of the Disclosure
The disclosure herein relates to mills for milling casings in a wellbore and specifically to mills that can be sheared when trapped (also referred to herein as stuck or lodged) in the wellbore to remove the service strings utilized to convey such mills in the wellbores.
2. Background of the Art
Wellbores are drilled in subsurface formations for the production of hydrocarbons (oil and gas). Modern wells can extend to great well depths, often more than 15,000 ft. A wellbore is typically lined with casing (a string of metal tubulars connected in series) along the length of the wellbore to prevent collapse of the formation (rocks) into the wellbore. A variety of devices are installed in the wellbore to produce the hydrocarbons from the formation surrounding the wellbore from one or more production zones. Sometimes it is necessary to mill a part of the casing to perform a downhole operation or for other reasons. The casing section remaining above the milled portion is sometimes removed from the wellbore. To perform a milling and retrieving operations, a tool (commonly referred to as a mill, with cutting members (also referred to as knives or blades) is typically conveyed into the casing by a service string to mill a certain length of the casing at a desired location. Sometimes the mill becomes trapped or lodged in the casing, such as in the case of cutting members not retracting after the milling operation or due to another downhole condition. Some mills include a shear mechanism on the upper section of or above the mill that allows an operator to over-pull the service string to cause the mill to separate from the service string and drop in the wellbore, which allows the operator to retrieve the service string to the surface. It is also desirable not to leave tools in the wellbore or minimize the size of such debris left or dropped in the wellbore so as to avoid performing secondary operations to remove tools left in the wellbore before performing operations needed at a later time or to not obstruct flow of fluids flowing through the wellbore or for other reasons.
The disclosure herein provides apparatus that provides, among other things, mills with shear mechanism that allows disconnecting cutting members from the mill when such mills are trapped in the wellbore.